A molecule is a group of two or more atoms that stick together. Molecules (MOLL-uh-cyools) are so small that nobody can see them, except with an electron microscope. Pretty much everything on Earth and other planets is made of molecules, and so is some of the dust in space.

The first molecules formed about 300,000 years after the Big Bang, or just under 15 billion years ago. They were the smallest kind of molecule - two hydrogen atoms joined together. As time went on, and supernovas from exploding stars shot out different kinds of atoms, different kinds of molecules formed and floated around in space. Because most of the atoms in space were hydrogen atoms, many of these molecules combined hydrogen with another kind of atom. So hydrogen combined with oxygen to make water molecules. Hydrogen combined with carbon to make hydrocarbons (what living things are built out of). Even before there were any planets, water and hydrocarbons were floating around in space on their own. Other molecules were made of heavier atoms, like silicon or gold. Still out in space, some hydrocarbons got together and formed bigger molecules called amino acids.

When the planets did form, the ones that were further away from stars, like Jupiter and Neptune, were made mostly of lighter molecules like water and hydrocarbons. Earth, which formed about 4.5 billion years ago, was closer to the Sun, and made mostly of heavier molecules like iron. A lot of silicon and other minerals also got to Earth, where they make up the rocks of the Earth's crust.

We're not sure how or when the water, hydrocarbons, and amino acids got to Earth. But once they were on Earth, the amino acids got together to make more and more complicated molecules - maybe first ribonucleic acids, then proteins. The biggest organic molecule today is DNA. Each molecule of DNA has more than two billion carbon atoms in it (plus a lot of other kinds of atoms too).

All molecules need some way to hold their atoms together. The two main ways that atoms hold together are

Today people can make new kinds of molecules in laboratories and factories. Some of the biggest molecules that people make are plastics, like what plastic garbage bags or plastic Legos are made of. Plastics are also hydro-carbon molecules. People also make new molecules for medicines.

To find out more about molecules, check out these books from Amazon.com
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